The present application hereby claims priority under 35 U.S.C. Section 119 on European application number EP 00128573.3, the entire contents of which are hereby incorporated herein by reference.
The invention generally relates to a method of decoating a parent body, preferably provided with an anti-corrosion coating, of a turbine blade.
Turbine blades, in particular gas turbine blades, are often provided with an anticorrosion coating for protection against corrosion and oxidation. Especially in the case of gas turbine blades which are used in a gas turbine at temperatures above 600xc2x0 C. or even above 1000xc2x0 C., such a protective coating is important for achieving a sufficiently long life.
Such a protective coating is usually made of a material of the group MCrAlX, where M stands for iron, cobalt or nickel, Cr stands for chromium, Al stands for aluminum, and X is selected from the group of yttrium, scandium, lanthanum and rare earths. For use at especially high temperatures, such a protective coating is often applied to a parent body of the turbine blade, the parent body including a nickel- or cobalt-base superalloy. In addition, a ceramic thermal-insulation layer may be applied to the anti-corrosion coating.
The coating wears out with time due to oxidation and corrosion; erosion and mechanical damage may also occur. In order not to have to exchange the turbine blades completely after a certain operating period, it is generally worthwhile restoring the protective coating. This xe2x80x9crefurbishmentxe2x80x9d first of all requires the careful removal of the old anti-corrosion coating from the turbine blade.
WO 93/03201 shows such a decoating process. Here, an old anti-corrosion coating in which, in particular, corrosion products are embedded is treated by cleaning and by subsequent application of an aluminide coating. With the subsequent removal of this aluminide coating, the anti-corrosion coating together with the corrosion products is also removed. This process is very effective, but comparatively complicated and expensive.
An object of the invention is to specify an effective and cost-effective method of removing an anti-corrosion coating from a turbine blade.
According to the invention, this object is achieved by, for example, a method of decoating a parent body, provided with an anti-corrosion coating, of a turbine blade. Preferably, a first, outer part, lying on the outside relative to the parent body, of the anticorrosion coating is removed abrasively by a water jet. Thereafter, a second, inner part, lying between the outer part and the parent body before the removal of the outer part, of the anticorrosion coating is removed chemically.
Such a method, for the first time, combines mechanical removal of an anti-corrosion coating by use of a water jet, with chemical removal. The mechanical removal is especially quick and thus cost-effective. However, removal of the anti-corrosion coating solely by use of the water jet could lead to damage to the parent body, which must as far as possible remain unaltered in its surface form, especially on account of aerodynamic requirements. Therefore only an outer part of the anti-corrosion coating is removed by the water jet. Further removal is subsequently effected via chemical attack.
A) The anti-corrosion coating has an average total coating thickness, the outer part preferably having an outer-part coating thickness which is at least 70% of the total coating thickness. The largest proportion of the anti-corrosion coating is therefore preferably removed abrasively via the water jet. It is also preferred that the outer-part coating thickness is at most 95% of the total coating thickness. This ensures that the water jet does not strike the parent body and cannot damage the latter as a result.
B) The inner part is preferably removed by using hydrochloric acid.
C) The water jet preferably strikes the anti-corrosion coating under a pressure level between 10-100 bar.
D) The anti-corrosion coating preferably includes MCrAlX, where M is selected from the group (iron, cobalt, nickel), Cr is chromium, Al is aluminum, and X is selected from the group (yttrium, scandium, lanthanum, rare earths). Such an anti-corrosion coating is especially effective at very high temperatures. During long-term stress, such an MCrAlX coating is subjected to a depletion of the beta phase. This depletion of the beta phase in the outer region of the anti-corrosion coating leads to a situation in which chemical attack alone, for removing the anti-corrosion coating, is only possible with difficulty and in a complicated manner. Especially in the case of such a beta-depleted anti-corrosion coating, the combination of the chemical decoating with previous abrasive, mechanical decoating is therefore especially advantageous.
E) The parent body preferably includes a nickel- or cobalt-base superalloy. Such an alloy is especially resistant to high temperatures, but is also more expensive than, for instance, high-temperature-resistant steels. Accordingly, the xe2x80x9crefurbishmentxe2x80x9d, that is the decoating and subsequent re-application of a new coating, is worthwhile, especially in the case of such a parent body.
F) After the chemical removal, the residual coating thickness of the anticorrosion coating is preferably determined. This may be done, for example, thermographically. In this way, the points on the parent body where there are still residues of the anti-corrosion coating are determined and the thickness of the residual coating regions is determined. Such remaining coating regions of the anti-corrosion coating which have a residual coating thickness greater than 5% of the original total coating thickness are then preferably also removed abrasively with the water jet down to a minimum thickness. In sections, therefore, comparatively thick coating regions are removed again by a water-jet treatment, although here the coating regions are not removed right down to the parent body but preferably only down to a minimum thickness in order to protect the parent body. Further chemical removal of remaining residual coating regions is then also preferably carried out.
G) The parent body is preferably single-crystalline or directionally solidified. Such a parent body has an especially high loading capacity under centrifugal forces and is produced in a comparatively complicated and expensive manner. Here, reprocessing of the anti-corrosion coating is especially appropriate economically.
H) The parent body preferably has a longitudinal extent greater than 20 cm. Especially in the case of such large turbine blades, conventional refurbishment is very time-consuming and thus expensive. Here, the combined treatment with a water jet and chemical removal leads to especially high cost advantages.
The embodiments according to paragraphs A) to H) may also be combined with one another in any desired manner.